The invention is directed generally to spirally-wound constructions for use in cross flow separation operations; these constructions are variously referred to as elements, cartridges and modules. More particularly, the invention is directed to such spirally-wound elements made of leaves in the form of folded sheets of polysulfone or polyethersulfone UF membranes, which may optionally carry interfacially created, more selective semipermeable membranes; these leaves are interleaved with sheets of feed passageway-providing material and permeate carrier material. Such elements have traditionally been made by strategically applying adhesive (referred to in the trade as “gluing”) to assemblages or lay-ups of such sheet-like materials while rolling about a porous tube to create a spirally-wound construction.
The earliest semipermeable membranes used for such separation operations were of the asymmetric, cellulose diacetate/triacetate type; however, in the past three decades, these membranes have been supplanted for many separation processes by asymmetric polysulfone or polyethersulfone UF membranes and by composite or thin film membranes wherein a more highly permeselective membrane has been coated onto a polysulfone base membrane or porous support. A dense active discriminating layer is often interfacially formed upon a more porous supporting or base layer; the dense layer is often a condensation polymer, such as a polyamide, which provides particularly desirable semipermeable characteristics. Although the more porous supporting layer can be any suitable polymeric material, polysulfones have frequently been used. Such a polysulfone layer having the desired pore size to support such an ultrathin interfacial layer is frequently cast upon a thin layer of nonwoven polyester felt backing or scrim material with which the polysulfone layer generally becomes very tightly attached. In the traditional spiral-wound construction, membrane leaves are formed by folding a long sheet having twice the final leaf length, and in some separations, polysulfone and polyethersulfone UF membrane, as well as composite sheet materials, have experienced occasional difficulties in the fold area where the UF membrane and/or thin interfacial membrane is folded upon itself. After use for some time, the fold region of the semipermeable membrane was found to have buckled and cracked, resulting in some leakage of feed solution being fed to the element through these cracks into the permeate carrier. Often a blister will form in the fold region or along edge regions of the membrane leaves, trapping feed solution or cleaning solution under the surface of the membrane; thus, even if the cracks do not leak through to the permeate side, they may create an unsanitary spot where bacteria can be harbored. Such is unsuitable in food and dairy process plants where products are being made for human consumption.
U.S. Pat. No. 4,842,736 recognized this problem at the fold and proposed an effective solution, teaching the application of flexible sealing material to the felt at the permeate output surface of the membrane material; such sealing material would penetrate and fill the interstices of the porous membrane support in the region of the fold eliminating flow in the region of the fold by blocking the output surface. Materials that were used for this purpose included polyurethane adhesives which were forced into the felt and then cured; alternatively, soft melt plastic ribbons were heated to essentially their melting point and driven into the interstices. A very similar solution to this problem of leakage at the fold was described in U.S. Pat. No. 5,147,541, and U.S. Patent Application No. 2004/0099598 further describes treatment along the fold line. U.S. Pat. No. 6,068,771 and U.S. Published Application No. 2003/0034293 disclose using vacuum to draw a polymeric adhesive into the edges of a spiral-wound membrane element.
Although these solutions solved the problem of leakage through cracking at a fold, it has been found that membrane leaf-folds which have been so treated to overcome the propensity for leakage through cracks may still experience other deficiencies when operated in environments where they are frequently subjected to harsh cleanings. This is particularly true in food and dairy installations where such spirally-wound elements are often cleaned daily, using cleaning solutions of a caustic or acidic character and/or which may contain high amounts of chlorine. In such regions where the downstream or permeate-output surfaces of such membrane sheet materials are sealed, e.g. in the fold regions, by a process such as one of those just mentioned above, caustic cleaners, for example, can penetrate through potential cracks, become absorbed in portions of the porous backing layers and sometimes create blisters by causing either the polysulfone to split from its substrate backing or the ultrathin layer to split from the polymeric porous base. Such regions also exist along the side and end edges of such membrane material where adhesive is traditionally applied so as to seal the edges of permeate carrier sheets (which provide the pathways adjacent each spirally disposed membrane leaf leading inward to the porous central collecting tube), and these seals will also prevent permeate passing through the active membrane surface in these edge regions from reaching the permeate carrier, as will also be the case in fold regions that have not cracked. It has now been found that liquids or solutions with relatively low osmotic pressure, e.g. DI rinse water, being pumped through the feed carrier windings will diffuse through or be absorbed within the semipermeable membrane in these edge areas, fill the porous region and sometimes cause local separation either of the polysulfone from its substrate or of the interfacial layer from the underlying polymeric base. This occurrence has now come to be referred to as osmotic blistering, and such blisters potentially occur along the glued edges of the membrane sheet lay-ups and in the region of the folds. When the elements are frequently cleaned and then rinsed with low osmotic pressure solutions, such as deionized water or the like, they will occasionally blister. Such blistering is unacceptable in the food and dairy industries, and a solution for this further problem has been sought for a number of years.